1. Field of the Invention
The present invention relates to a magnetic film, a method of magnetization inversion of a magnetic film, a mechanism of magnetization inversion of a magnetic film and a magnetic random access memory.
2. Description of Related Art
In recent years, it is expected that a magnetic random access memory (MRAM) is used instead of many conventional solid state memories. The MRAM is a high-speed nonvolatile memory using the magnetoresistive effect based on the electron spin dependent conduction phenomenon in the nano-technology. Especially, research reports recently issued have substantiated the high practicability of the MRAM which, using the spin tunneling magnetoresistive (TMR) effect, can produce a large read signal and find advantageous application for high-density recording or high-speed read operation.
In the MRAM, a magnetoresistive film is used as a memory element. The magnetoresistive film is basically configured of a sandwich structure, as shown in FIG. 5, in which two magnetic films B, C are stacked and sandwich a nonmagnetic film A. The resistance value of the MRAM is varied with the direction of magnetization of the transition metal atoms of the two magnetic films B, C (hereinafter simply referred to as “the direction of magnetization of the magnetic films”) formed in contact with the nonmagnetic film A. Specifically, when the direction of magnetization is the same (parallel) for the two magnetic films B, C, the resistance value is comparatively small, while when the direction of magnetization is opposite (antiparallel), the resistance value is comparatively large. The MRAM reads the information using this characteristic. The information is written, for example, on the assumption that “0” is involved when the directions of magnetization of the two magnetic films B, C are parallel to each other, while “1” is indicated when the directions are antiparallel. The information is written in such a manner that the magnetic film C is magnetized in the direction corresponding to the information to be recorded, while the direction of magnetization of the other magnetic film B is fixed. When a constant voltage is applied to a magnetoresistive film having recorded therein “0”, a comparatively large current flows in the particular magnetoresistive film, while a comparatively small current flows when “1” is recorded in the magnetoresistive film. This difference in current value is detected by a sense amplifier thereby to judge whether the recorded information is “0” or “1”. In this way, the information can be read out.
In the case of the MRAM using an in-plane magnetized film as a magnetic film, the reduction in size of the memory element to improve the recording density poses the problem that the information cannot be held due to the effect of the demagnetizing field or the curling of magnetization at the end surface. Specifically, the magnitude of the saturated magnetization of the magnetoresistive film such as NiFe/Cu/Co is not less than about 800 [emu/cc]((1/4n)×104 [emu]=1 [Wb/m2]). Once the width of the memory element is reduced to the order of submicrons, the demagnetizing field increases due to the approach of the magnetic pole of the film end surface, with the result that the spin is reversed at the film end surface and oriented in the direction parallel to the side surface.
A solution to this problem of the in-plane magnetized film may be to form a rectangular magnetic film. This method, however, cannot reduce the size of the memory element and therefore cannot be expected to sufficiently improve the recording density. A method to avoid this problem by use of a vertical magnetized film has been proposed as described in, for example, Japanese Patent Application Laid-Open No. 11-213650 (US6219275). The magnitude of the saturated magnetization for the vertical magnetized film is controlled to a maximum of about 300 [emu/cc], and the energy of the demagnetizing field is smaller than the vertical magnetic anisotropy constant. Even when the width of the memory element is reduced, therefore, no approach of the magnetic pole of the film end surface occurs and the demagnetizing field is not increased. For this reason, a MRAM smaller in size than the MRAM using a memory element formed of an in-plane magnetized film can be implemented. An amorphous alloy film of the rare-earth metal or the transition metal is suitable for the vertical magnetized film used as a memory element of MRAM. The rare-earth metals generally used for this purpose include Gd and Tb, while the transition metals include Fe and Co.
In recording information in the MRAM, a write current is supplied to a write line arranged near a memory element (magnetoresistive element), and by use of the magnetic field generated by the write current, one of the magnetic films is magnetized in the direction corresponding to the information to be recorded. The magnetic field currently required to reverse the direction of magnetization of the magnetic film (magnetic flux reversal) is about several tens of [Oe] (1[Oe]=79.6 [A/m]). For the magnetic field of this size to be applied to the memory element, however, a comparatively large write current is required to be supplied to the write line, and depending on the sectional area of the write line, the current density is exceeded at which the wire may be broken by migration or the like. That is, although a compact MRAM can be realized by use of a memory element formed of a vertical magnetized film, the limit of the write current that can be supplied to the write line poses the new problem that information cannot be recorded in the memory element, while at the same time increasing the power consumption. This problem can be solved by increasing the sectional area of the write line at the sacrifice of running counter to the requirement to reduce the size of the memory as a whole on the one hand and the failure to solve the problem of an increased power consumption on the other.